The Anatomy of Chlamydoselachus 



385 



mirrored image of the dorsal bundle; but it is expressed, with some modifications, in the 

 trunk region also. These modifications have to do with (a) the incipient separation of 

 a superior oblique muscle from an inferior oblique, and (b) the inroUmg of the ventral 

 column of the ventrolateral bundle to form the muscles of the tropeic folds — structures 

 peculiar to Chlamydoselachus. In Heptanchus (Text'figure 56) conditions are not so 

 simple, for there is a small middle oblique muscle and there is considerable overlapping 

 of the middle and superior oblique muscles by the inferior oblique. The figure for Chlamy- 

 doselachus is drawn from a rather small specimen, 1330 mm. long. The figure for Heptan- 

 chus is from a specimen 900 mm. long. 



Since the abdominal or tropeic folds are structures peculiar to Chlamydoselachus, 

 their musculature is entitled to further consideration. The superficial appearance of the 

 tropeic folds has been described, in three adult specimens and six large embryos, by 



Text-figure 57- 

 Transverse section showing the tropeic 

 folds (x 1) of an adult Chlamydo- 

 selachus. This section was taken eight 

 inches in front of the pelvis. 

 After Garman, 1885.2, Fig. B, pi. XX. 



Gudger and Smith (1933). The internal structure of the abdominal folds in a single 

 adult specimen has been figured by Garman (1885.2) in his Figs. A and B, pi. XX— the 

 latter figure being reproduced as my Text'figure 57- Concerning these figures Garman 



(p. 21) says: 



One of the folds is seen to hang below each of the large abdominal vessels. The vessels 

 are parallel or nearly so. Between them are two muscular bands, one to each fold. Each 

 band is nearly an inch in width, very thin at its lower edge, and near one-fifth of an inch thick 

 toward the rounded upper edge, between the veins. The fiber in these tropeic ... or keel 

 muscles differs from that in the walls of the flank in being coarser in the bundles and plates, 

 and more loosely put together. Apparently the keel muscle corresponds to the rectus 

 abdominis of lower vertebrates. 



Carman's figures readily suggest that the keel muscle is derived during development 

 by an infolding of the musculature of the ventral body wall. In order to test this hypoth- 

 esis I have prepared transverse serial sections from a segment of the ventral abdominal 

 wall excised from a 210-mm. male embryo. In this specimen the distance from pectoral 

 fin to pelvic girdle is 55 mm. The segment comprised the region extending from 10 mm. 

 to 20 mm. in front of the pelvic fins. A drawing (Text-figure 58) was made from a section 

 taken approximately 15 mm. from the pelvic fins — corresponding very nearly to the region 

 (200 mm. in front of the pelvic girdle) figured by Carman for his large adult specimen. 

 In my sections I have found some further indications of the manner of origin of the muscle 



